1. Field of the Invention
The present invention relates to a wavelength locker for locking the wavelength of a laser beam outputted from a laser source, at a specific wavelength and, more particularly, to a wavelength locker capable of arbitrarily setting a specific wavelength to be locked. The invention relates further to a multi-constant wavelength light source and a wavelength-division multiplexing light source using that wavelength locker. Further related is a wavelength discriminating apparatus for discriminating the wavelength of a laser beam to identify what the wavelength is of.
Especially, the WDM light source to be used in the WDM system has to output a laser beam in a plurality of wavelengths, whose spaces have to be given a grid determined for each channel (as will be abbreviated into the xe2x80x9cchxe2x80x9d) according to the advice of ITU-T. For this necessity, there has been investigated and developed the WDM light source.
2. Description of the Related Art
In the case of a WDM optical communication system, for example, in the prior art for wavelength-multiplexing eight optical signals, the WDM light source is provided with eight semiconductor lasers for oscillating laser beams with wavelengths different from each other. As these semiconductor lasers, moreover, there is used a distributed feedback laser (as will be abbreviated into the xe2x80x9cDFBxe2x80x9d) and a distributed Bragg reflector laser (as will be abbreviated into the xe2x80x9cDBRxe2x80x9d).
On the other hand, these semiconductor lasers are designed on the pitch of a diffraction grating so that a single-mode laser beam of a specific wavelength may be oscillated in a steady state. The semiconductor lasers will not always oscillate in the specific wavelength when they are ignited. In a steady state, too, some fluctuations are present so that the oscillation wavelength is not always locked at the specific wavelength.
In order to lock the oscillation wavelength at the specific wavelength, therefore, a wavelength locker is employed in the WDM light source.
In FIG. 38A, a laser beam outputted from a DFB laser 710 is inputted into a coupler 711 in a wavelength locker 700 for branching an input beam into two. After a portion of the laser beam was branched, the remaining laser beam is outputted from the wavelength locker 700. The DFB laser 710 is a semiconductor laser for oscillating, when in the steady state, a laser beam having a wavelength corresponding to a ch0.
In the wavelength locker 700, a portion of the laser beam branched by the coupler 711 is inputted into and branched by a coupler 712 for branching an input beam into two. One laser beam branched by the coupler 712 is inputted through a Fabry-Perot Etalon filter (as will be abbreviated into the xe2x80x9cET filterxe2x80x9d) into a first photodiode (as will be abbreviated into the xe2x80x9cPDxe2x80x9d) 714 for outputting an electric current in accordance with a light-intensity so that its intensity is detected by the PD 714. This first PD 714 has an output value PDo1. On the other hand, the other laser beam branched by the coupler 712 is inputted into a second PD 715 for outputting an electric current in accordance with the light-intensity so that its intensity is detected by the PD 715. This second PD 715 has an output value PDo2.
The wavelength of the ET filter 713 for giving the maximal value of a light transmittance is so set that the value PDo1 standardized with the PDo2 in the wavelength to be locked, that is, the value PDo1/PDo2 may become a target value of 0.5.
And, a controlling CPU 716 receives those values PDo1 and PDo2 and sends a control signal for locking the oscillation wavelength of the DFB laser 710 at a specific wavelength in accordance with those detected values, to the DFB laser 710.
The wavelength locker 700 thus constructed operates in the following manners to lock the oscillation wavelength of the DFB laser 710 at the ch0.
After having ignited the DFB laser 710, the controlling CPU 716 receives the values PDo1 and PDo2 and calculates the value PDo1/PDo2. When this value PDo1/PDo2 is larger than the target value of 0.5, moreover, the controlling CPU 716 controls the DFB laser 710 by adjusting the drive current or temperature of the DFB laser 710 so that the oscillation wavelength may become longer. When the value PDo1/PDo2 at the time of igniting the DFB laser 710 is smaller than the target value of 0.5, on the other hand, the controlling CPU 716 controls the DFB 710 so that the oscillation wavelength may become shorter. Thus, the DFB laser 710 is controlled so that the value PDo1/PDo2 may always become 0.5, and the oscillation wavelength is locked at the ch0.
By the way, the controlling CPU 716 controls the oscillation wavelength merely by comparing the magnitudes of the value PDo1/PDo2 and the target value of 0.5. So, when the DFB laser 710 is ignited at wavelengths of points a, b, c and d of FIG. 38B, the oscillation wavelength can be locked at the desired ch0, but when the DFB laser 710 is ignited at wavelengths of points e and f, the oscillation wavelength is locked at wavelengths other than the ch0.
Considering the wavelength range at the ignition time of the DFB laser, therefore, the wavelength locker is designed to one specific wave to be locked.
Here, the wavelength range within which the wavelength locker can lock the oscillation wavelength of the laser at a desired wavelength with respect to the wavelength at the laser igniting time will be called the xe2x80x9clocking rangexe2x80x9d. Moreover, this locking range is determined by the free spectrum range (as will be abbreviated as xe2x80x9cFSRxe2x80x9d) of the ET filter and can be widened by enlarging the FSR.
When the DFB laser 710 in FIG. 38A is replaced by a tunable wavelength laser capable of oscillating a single mode and making the oscillation wavelength continuously variable, on the other hand, there is employed a wavelength locker capable of oscillating a specific wavelength from a plurality of wavelengths to lock the oscillation wavelength.
In FIG. 39A, on the other hand, the construction of a wavelength locker 750 is similar to that of FIG. 38A excepting that the ET filter 713 of FIG. 38A is replaced by an ET filter 754 having an FSR conforming to the wavelength space to be locked, and its description will be omitted.
This locking range of the wavelength locker 750 is determined with the wavelength space of the WDM system, as shown in FIG. 39B, when it is used in the WDM tunable wavelength laser. This is because the ET filter is a periodic filter so that the value PDo1/PDo2 takes an identical value for every constant periods and because a controlling CPU 757 merely receives the values PDo1 and PDo2 so that it cannot discriminate points g and h, for example.
The locking range is about xc2x130 GHz in the wavelength locker which is used when each ch is located having wavelength spaces of 0.8 nm.
Thus, the WDM light source is constructed to include semiconductor lasers of different oscillation wavelengths in a number corresponding to that of wavelengths to be multiplexed. When preparatory semiconductor lasers are to be prepared for a breakage of the semiconductor lasers, therefore, they have to be prepared for every semiconductor lasers of different oscillation wavelengths. In the WDM light source for thirty two waves, for example, thirty two ordinary semiconductor lasers of different oscillation wavelengths are provided for every thirty two ch so that thirty two semiconductor lasers have to be individually prepared for the preparatory ones.
This means that a number of semiconductor lasers will become necessary when the degree of multiplicity increases with the future increase in the traffic.
By preparing a tunable wavelength laser capable of outputting a plurality of wavelengths, on the other hand, the number of semiconductor lasers could be reduced. In this case, however, there is no suitable wavelength locker.
Specifically, this light source outputs the plurality of wavelengths so that it cannot be easily coped with by the one-wave wavelength locker which is designed for one specific wave to be locked.
Since the locking range is narrow when the wavelength locker is used to cover a plurality of wavelengths, on the other hand, the tunable wavelength laser is locked at an unexpected wavelength when it is ignited over that locking range. In FIG. 39B, for example, the tunable wavelength laser is ignited to oscillate a wavelength of a ch1. When the tunable wavelength laser is ignited at the point h, the oscillation wavelength is locked at a ch2.
When the tunable wavelength laser capable of outputting the wavelengths is employed, moreover, an optical signal in operation of an optical communication system connected with the tunable wavelength laser is adversely affected if the laser beam is outputted into the optical communication system before the locking at the specific wavelength.
An object of the invention is to provide a wavelength locker and a WDM light source which have a wider locking range than of the wavelength locker in the prior art.
Another object of the invention is to provide a wavelength locker and a WDM light source, each of which can cope with a plurality of wavelengths.
Still another object of the invention is to provide a wavelength locker and a WDM light source which can lock an oscillation wavelength at a desired wavelength no matter what wavelength they might be ignited with at the start of an emission.
A further object of the invention is to provide a laser light source which can output a laser beam after the wavelength of the laser beam is locked at the desired wavelength.
A further object of the invention is to provide a wavelength discriminating apparatus making use of a major portion of a wavelength locker according to the invention for detecting the wavelength of a beam.
The aforementioned objects are achieved by a wavelength locker, which comprises a filter, a detecting part for detecting the intensity of a light through the filter, and a controlling part for controlling the wavelength of the light to a desired wavelength in accordance with the output of the detecting part.
In this wavelength locker, for example, the filter is exemplified by an interferometer, and the controlling part may decide the evenness/oddness of the desired wavelength when numbers are sequentially assigned to a plurality of wavelengths.
In this wavelength locker, on the other hand, the plurality of wavelengths are equally spaced so that the several characteristics of the filter may be determined according to the equal space and so that the controlling part may decide the evenness/oddness of the desired wavelength or may store a plurality of target values corresponding to the wavelengths when numbers are sequentially assigned to the wavelengths.
In this wavelength locker, moreover, the filter may be exemplified by a plurality of filters having different FSRs so as to provide a plurality of locking ranges, and the controlling part may lock the wavelength stepwise by using those locking ranges.
In this wavelength locker, moreover, the plurality of wavelengths are equally spaced so that the filter may be exemplified by a plurality of filters having several characteristics filter determined according to the equal spaces.
On the other hand, the WDM light source is constructed by providing such wavelength locker with a light source capable of oscillating a beam having a plurality of wavelengths. Alternatively, the WDM light source is, for example, constructed by providing such wavelength locker with a shading part for changing a light transmittance in accordance with the wavelength of a beam to be outputted from the light source. This light source is suited for a preparatory light source.
Moreover, the wavelength discriminating apparatus is constructed of a plurality of filters which have several characteristics determined according to equal spaces.